Homework Help: Force needed to push box down incline plane

1. Sep 16, 2008

Physics_girl1

1. The problem statement, all variables and given/known data
a) if a force of 86N parallel to the surface of a 20 degree incline plane will push a 120N block up the plane at constant speed, what force parallel to the plane will push it down at constant speed?

2. Relevant equations
$$\Sigma$$F(X)=F(A)-F(fr)-mgcos$$\vartheta$$=0
$$\Sigma$$F(Y)=F(n)-mgsin$$\vartheta$$=o
where F(A)= force applied, F(fr)= friction force, F(n)= normal force, mg=weight, F(X)= sum of forces in x direction, and F(Y)- sum of forces in y direction

3. The attempt at a solution
normal force=41.0424
friction force=26.73

Last edited: Sep 16, 2008
2. Sep 16, 2008

Physics_girl1

my coefficient of friction for this problem=.652

3. Sep 16, 2008

Physics_girl1

4. Sep 17, 2008

ritwik06

Hi
Writing out the equation for the force to push up the block with constant speed, i.e. net force is zero.
120 sin 20(component of weight along incline) +120 cos 20 * u(Force of friction, u is the coefficient)=86
solve for u
u=0.399
Now for the second part:
120 sin 20 +F=120 cos 20 *u
F=120 sin 20-120 cos 20 *u
F=3.9 N
dos this help?????